The present invention relates generally to methods and apparatus for dispensing coating material on a moving substrate.
The method and apparatus of the invention may be applied in the coating of a paper or cardboard web in which case the invention relates to a coating arrangement for use in paper making. The invention may also be applied in the dispensing of coating material on the surface of a roll, such as in a sizing press, from which the coating material is then transferred, in a throat between press rolls, onto a paper or cardboard web passing through the sizing press. In this case, the invention relates to sizing press arrangements used in paper making.
More particularly, the invention relates to methods and apparatus for dispensing coating material on a moving substrate in which coating material is introduced under pressure into coating apparatus before the coating nip and wherein the coating material is directly applied on the surface of the moving substrate through a slit nozzle provided in the nip.
A known arrangement for dispensing coating material on a moving substrate uses the so-called doctor coating method. Due to its numerous advantages, the doctor coating method is practically the only coating procedure being used, particularly in high-speed coating machines. One of the most important advantages of this method is that a smooth paper (or cardboard) surface is obtained which is very important for obtaining a finished product having good printing characteristics.
In the doctor coating method, an excess quantity of coating material is introduced on the paper web which is supported by a rotating roll whereupon an elastic doctor blade disposed against the web functions to scrape off a portion of the coating material on the web surface in order to leave a desired quantity of coating material on the web in the form of a uniform layer. The portion of the coating material which is scraped off by the doctor is saved and subsequently reintroduced into the coating apparatus.
The main drawback of the doctor coating method is the difficulty in controlling the quantity of the coating material being applied on the substrate, e.g. the web. This is a result of the fact that several factors influence the quantity of coating material that is applied on the web, namely, the rheologic properties of the coating material, the geometric orientation of the coating doctor with respect to the web being coated, i.e., the so-called doctor angle, and the loading of the coating doctor, i.e., the pressure applied by the doctor on the web.
Since it is important to return the portion of the coating material which is scraped off by the doctor to the supply of fresh coating material being applied to the web in order to avoid loss of coating material, the coating material must have good flow properties. On the other hand, this necessarily makes it difficult to increase the dry matter content of the coating material which should be relatively high in order to yield a high quality coating. For example, migration of the bonding agent in the coating will be reduced when the coating material has a high dry matter content. Moreover, since a paper web must be conducted through drying apparatus subsequent to the coating operation in order to remove moisture from the web, a high dry matter content of the coating material is also advantageous from the viewpoint of energy efficiency.
Two different doctor coating methods are generally employed in coating paper or cardboard which essentially differ from each other with respect to the angle which the coating doctor forms with the opposing web roll being coating. In the so-called low-angle coating method, the angle between the doctor and an opposing web roll is only a few degrees. On the other hand, in the so-called high-angle coating arrangement, the angle which the doctor forms with an opposing web roll is generally several tens of degrees.
The low-angle coating arrangement for coating a cardboard web is advantageous in that it provides a coating layer having relatively uniform thickness which is important, for example, to ensure uniform absorption of printer's ink. Further, any impurities present in the coating material and/or the cardboard web will by-pass the doctor without adhering to it thereby avoiding the formation of doctor stripes or streaks.
On the other hand, the low-angle technique has the drawback that it is difficult to control the coating material quantity since the amount of coating material deposited on the web changes significantly when the relatively small angle between the doctor and the surface of the opposing roll is changed even minimally in order to regulate the coating quantity.
On the other hand, the high-angle coating arrangement is advantageous when small quantities of coating material are being applied, such as less than about 12 g/m.sup.2, in that relatively good coating material quantity control can be achieved. On the other hand, it is difficult to control coating material quantities when larger coating material quantities are being applied since the doctor will initially merely fill the irregularities in the web.
The most significant drawback of the arrangement wherein a coating is applied onto a surface of a roll in a sizing press for transfer onto a paper or cardboard web is that production rates are limited and the range of viscosity of the sizing agents which can be employed are also limited.
Another known arrangement for coating paper and cardboard is the extruder technique in which coating material is introduced onto the web by passing the coating material through a slit under pressure as the web passes by the slit. In this arrangement, the quantity of coating material dispensed onto the web is determined by the volumetric flow rate of the coating material supplied to the slit.
The main drawback of this conventional technique is that the coating quantity profile cannot be precisely adjusted since the distance of the scraping wall of the slit from the web cannot be locally adjusted. For example, if the opposing roll surface has irregularities in its profile due, for example, to wear, a poor coating quantity profile will result. In mineral pigment coating, the fixed scraper wall undergoes rapid wear. However, the replacement of this precision machined, relatively massive wall would require unreasonable expenditures.
Regarding the state of the art, reference is made to U.S. Pat. Nos. 2,946,307, 2,970,564 and 4,357,370. Briefly, an arrangement is disclosed in U.S. Pat. No. 2,946,307 in which a roll surface or a gate roll over which one surface of a paper web runs is coated using a steel rod while the other web surface is coated using a steel rod which at the same time presses the coating from the roll onto the contacting surface of the paper web. U.S. Pat. No. 4,357,370 discloses an arrangement wherein a roll surface is coated using a suitable applicator while the non-contacting side of the paper web is coated using a short delay doctor.